1. Field of the Invention
The present invention relates to a driving method of an ink-jet recording head in which thermal energy is made to act on ink and ink is discharged on the basis of a bubble generated thereby, and an ink-jet recording apparatus for performing the recording method.
2. Related Background Art
An ink-jet recording method, in which ink is heated to generate a bubble, ink is discharged on the basis of the generation of the bubble, and it is made to adhere onto a medium to perform image formation, has the advantages that high-speed recording is possible, the recording quality is relatively high, and the generated noise level is low.
In addition, this method has many superior advantages such as color image recording is relatively easy, recording can be performed even on a plain paper or the like, miniaturization of apparatus is feasible, and further, because the discharge outlets of a recording head can be disposed at a high density, a high-resolution and high-quality image can be recorded at a high speed. A recording apparatus using this method has been used as information output means in a copier, a printer, a facsimile, or the like,
The general construction of a recording head, in which such an ink-jet recording method is performed, comprises discharge outlets for discharging ink, ink flow passages communicating with them for ink supply and electrothermal transducing elements (heating elements) provided within those ink flow passages for generating thermal energy. Each of the heating elements is generally made of a thin-film resistance element. Thermal energy is generated by electrifying each of the heating elements through electrode wiring in a pulse manner (applying drive pulse).
When an overheated liquid layer for storing foaming energy is to be formed in ink by applying thermal energy to ink near the heating element, in a case that the state of the heating element surface (ink heating surface) has partially changed due to scorching of ink, damage, or the like, or impurities or gas has mixed in the ink, heat is hindered from flowing into the overheated liquid layer because a foaming nucleus has been generated in an early stage of heating. As a result, unevenness of foaming start times in the ink on the heating element surface occurs. Because such unevenness of foaming start times causes unevenness of foaming energy of bubbles, there may arise a change in the discharge quantity or discharge velocity of ink to deteriorate image quality.
Therefore, in order to provide an ink-jet recording head good in reproducibility of discharge characteristics of ink droplets such as discharge velocity, it is required to decrease unevenness of foaming start times. For this purpose, it is important to increase the temperature rise rate dT (t0) at the foaming time t=t0. The reason for this will be described below with reference to FIG. 16.
Although foaming probability of ink depends on the temperature distribution in the ink, it changes from 0 to 1 when the temperature T of the portion at the highest temperature in the ink shifts from the lower temperature side to the higher temperature side of the temperature range T1 less than T less than T2 near the overheat limit. FIG. 16 is a diagram showing a change in the temperature T of ink in contact with a heating element surface being at the highest temperature. When the temperature rise rate at the foaming time t=t0 is dT (t0), unevenness xcex94t of foaming times is given by:                               Δ          ⁢                      xe2x80x83                    ⁢          t                ≈                              T2            -            T1                                d            ⁢                          xe2x80x83                        ⁢                          T              ⁡                              (                                  t                  ⁢                                      xe2x80x83                                    ⁢                  o                                )                                                                        (        1        )            
Therefore, for decreasing the unevenness xcex94t of foaming start times, the temperature rise rate dT (t0) should be increased.
For decreasing xcex94t, it is known that rapid heating is effective in which the temperature of ink near a heating element surface is rapidly heated to the homogeneous nucleation temperature before a foaming nucleus is generated at the boundary surface (or interface) between ink and the heating element surface (A. Asai et al., xe2x80x9cBubble Generation Mechanism in the Bubble Jet Recording Processxe2x80x9d, Journal of Imaging Technology, Vol. 14, pp. 120-124, 1988).
In case of performing rapid heating, the shorter applying time of the drive signal causes a lesser heat quantity that can fully flow into the ink, at a point in time, and so a lesser thickness of ink (overheated liquid layer) in such an overheated state that a foaming nucleus can grow to a bubble results.
A large quantity of evaporative latent heat required by the overheated liquid layer that has started the homogeneous nucleation in rapid heating is mainly supplied from the heating element side. But, there is ink at a low temperature outside the overheated liquid layer, and a large quantity of heat flows out of the thin overheated liquid layer to the ink side outside the overheated liquid layer, which is at a great difference in temperature from the overheated liquid layer. For this reason, if rapid heating is performed with shortening the applying time (heating time) of the drive signal, the essentially required quantity of evaporative latent heat cannot fully be supplied to the overheated liquid layer.
Therefore, if the heating time is shortened, foaming energy decreases, and it becomes hard to obtain a sufficient discharge velocity. (A. Asai, xe2x80x9cBubble Dynamics in Boiling Under High Heat Flux Pulse Heatingxe2x80x9d, J. Heat Transfer, Vol. 11B, pp. 973-978, 1991; Mitsuya et al., xe2x80x9cNucleus Boiling and Ink Discharge Characteristics in Ultra-rapid Heatingxe2x80x9d, Japan Hardcopy ""96, A-40)
As a result, when rapid heating is performed with a shortened heating time, xe2x80x9cinitial discharge performancexe2x80x9d is reduced and there is the possibility of no discharge in the worst case. (In case of performing no ink discharge for a certain time after an ink droplet is discharged through a nozzle, when an ink droplet is next discharged through the nozzle, trouble may arise that stable discharge cannot be performed due to an increase in viscosity of ink, and so printing falls into disorder. The discharge performance of the next droplet is referred to as xe2x80x9cinitial discharge performancexe2x80x9d.)
Besides, unevenness in resistance of the thin film resistance bodies of recording heads or unevenness in film thickness of protection layers formed on the thin film resistance bodies, which has not been at issue in conventional driving methods, readily results in unevenness in thickness of overheated liquid layers of the recording heads. This may cause unevenness in discharge quantity, discharge velocity, or the like, of the recording heads. Similarly, if there is a change in resistance of a thin film resistance element while foaming is repeated, it causes a change in discharge characteristics of the same recording head.
As described above, in such a driving method of a rapidly-overheated region in which unevenness of foaming start times can be reduced by rapid heating but foaming energy decreases, the discharge characteristics of recording heads may be unstable and uneven due to small foaming energy, which may deteriorate image quality.
The present invention has been made to solve the above problems, and aims to provide a driving method of an ink-jet recording head capable of performing stable ink discharge, wherein:
(1) unevenness of foaming start times is small to perform stable foaming; and
(2) foaming energy is great to ensure a sufficient discharge quantity and a sufficient discharge velocity, and to provide a recording apparatus in which such recording method is performed.
A driving method of an ink-jet recording head according to the present invention to attain such objects is a driving method of an ink-jet recording head that comprises a discharge outlet for discharging ink, an ink flow passage communicating with said discharge outlet, and a heating element for heating ink in said ink flow passage by applying a drive signal so as to generate a bubble, said head discharging ink through said discharge outlet on the basis of the generation of said bubble, wherein
said drive signal comprises a first drive signal for storing foaming energy in ink, and a second drive signal for generating a bubble in ink, and
a bubble is generated by applying, to said heating element, the drive signal in which:
when the time from application start of said second drive signal to bubble generation is t=xcex4t, and the boundary foaming time at which foaming energy decreases in case of generating a bubble only by said second drive signal without applying said first drive signal, is t=ts, xcex4t and ts satisfy the relation:
xcex4t less than ts;
and, when the applying time of said first drive signal, which is the difference in time from the time at which application of said first drive signal is started, to the time at which said second drive signal is started, is t1, the applying time of said second drive signal is (t2-t1), and the heating quantity (or calorific quantity) of said heating element by the drive signal is Q(t), t1, t2, and Q(t) satisfy:             1      t1        ⁢                  ∫        0        t1            ⁢                        Q          ⁢                      (            t            )                          ⁢                  xe2x80x83                ⁢                  ⅆ          t                       less than             1              δ        ⁢                  xe2x80x83                ⁢        t              ⁢                  ∫        t1        t2            ⁢                        Q          ⁢                      (            t            )                          ⁢                  xe2x80x83                ⁢                              ⅆ            t                    .                    
Or, it is a driving method of an ink-jet recording head in which heat is generated by applying a drive signal to a heating element, and this heat is given to ink to generate a bubble and discharge ink through a discharge outlet, wherein
said drive signal comprises a first drive signal for storing foaming energy in ink, and a second drive signal for generating a bubble in ink, and
said second drive signal of a signal time shorter than the boundary foaming time ts at which foaming energy decreases in case of performing foaming only by said second drive signal, is used, and said first drive signal for compensating a decrease in said foaming energy is applied prior to said second drive signal.
In each of the above methods, when the time at which a bubble is generated by said second drive signal is t=xcex4t, the temperature rise rate at this time is dT(xcex4t), the boundary foaming time at which foaming energy decreases in case of generating a bubble only by said second drive signal without applying said first drive signal is t=ts, and the temperature rise rate at this time is dT(ts), each temperature rise rate may satisfy:
dT(xcex4t) greater than dT(ts).
Said first drive signal may be for increasing the thickness of an overheated ink layer in ink receiving heat from said heating element.
The surface temperature of said heating element before applying said second drive signal may be heated to the boiling temperature or higher by said first drive signal.
When the time from application start of said second drive signal to bubble generation is t=St, the time at which a bubble is generated by said second drive signal is t=St, the boundary foaming time at which foaming energy decreases in case of generating a bubble only by said second drive signal without applying said first drive signal is t=ts, the boiling point of ink is Tb, the foaming temperature is Tg, and the temperature of ink before applying said first drive signal is Tamb, xcex4t may satisfy:       δ    ⁢          xe2x80x83        ⁢    t     less than                     Tg        -        Tb                    Tg        -        Tamb              ·          ts      .      
The ratio J1/J0 of the foaming energy J1 of a bubble formed only by said second drive signal without applying said first drive signal, to the foaming energy J0 of a bubble formed by said first and second drive signals, may satisfy:
J1/J0xc3x97100≲50 (%)
The heating quantity of said heating element by said second drive signal may be equal to or more than the heating quantity of said heating element at the boundary foaming time t=ts at which foaming energy decreases in case of generating a bubble only by said second drive signal without applying said first drive signal.
Said ts may be the boundary foaming time when the life of a bubble reduces.
Said ts may be the boundary foaming time when the discharge velocity reduces.
Said first and second drive signals may be a continuous signal.
A resting period may be interposed between said first and second drive signals.
Said first drive signal may comprise a plurality of pulses, and the resting periods between said pulses may gradually become longer.
An ink-jet recording apparatus according to the present invention is an ink-jet recording apparatus to perform recording using an ink-jet recording head that comprises a discharge outlet for discharging ink, an ink flow passage communicating with said discharge outlet, and a heating element for heating ink in said ink flow passage by applying a drive signal to generate a bubble, said head discharging ink through said discharge outlet on the basis of the generation of said bubble;
said apparatus having a first drive signal for storing foaming energy in ink, and a second drive signal for generating a bubble in ink;
said apparatus comprising drive signal supply means for applying, to said heating element, said drive signal in which:
when the time from application start of said second drive signal to bubble generation is t=xcex4t, and the boundary foaming time at which foaming energy decreases in case of generating a bubble only by said second drive signal without applying said first drive signal is t=ts, xcex4t and ts satisfy the relation:
xcex4t less than ts;
and, when the applying time of said first drive signal, which is the difference in time from the time at which application of said first drive signal is started, to the time at which said second drive signal is started is t1, the applying time of said second drive signal is (t2- t1), and the heating quantity of said heating element by the drive signal is Q(t), t1, t2, and Q(t) satisfy:             1      t1        ⁢                  ∫        0        t1            ⁢                        Q          ⁢                      (            t            )                          ⁢                  xe2x80x83                ⁢                  ⅆ          t                       less than             1              δ        ⁢                  xe2x80x83                ⁢        t              ⁢                  ∫        t1        t2            ⁢                        Q          ⁢                      (            t            )                          ⁢                  xe2x80x83                ⁢                              ⅆ            t                    .                    
Or, it is an ink-jet recording apparatus in which heat is generated by applying a drive signal to a heating element, and this heat is given to ink to generate a bubble and discharge ink through a discharge outlet, said apparatus comprising:
signal supply means for applying said drive signal to said heating element, said drive signal comprising a first drive signal for storing foaming energy in ink, and a second drive signal for generating a bubble in ink, said second drive signal having a signal time shorter than the boundary foaming time ts at which foaming energy decreases in case of performing foaming only by said second drive signal, said first drive signal being applied prior to said second drive signal so as to compensate for a decrease in said foaming energy.
Also in these ink-jet recording apparatuses, each of the above features addable to the above ink-jet recording head driving methods may be added.